1. Field of the Invention
The present invention relates to an improved method for welding a metallic fin to a metallic tube in the manufacture of a heat exchanger tube having a heat exchange surface that is increased by fins. More particularly, the present invention relates to such an improved method in which heat induced in the tube by resistance to electrical current flow during the welding process is supplemented with heat from a directed laser beam.
2. The Prior Art
In the manufacture of a finned heat exchanger tube, a metallic strip or fin is helically wound or wrapped on the outer surface of a rotating metallic tube to form the fins. The fin is fed to the tube surface under tension, with the bottom edge of the fin being welded to the tube.
High frequency welding, in which heat is produced in the metallic fin and tube by resistance to electrical current flow and the pressure of the heated fin under tension against the heated tube provides the compression to effect forge welding, is used for welding the fin to the tube. Electrical current flow is induced by a high frequency electromagnetic field established between electrodes at the fin and at the tube, both electrodes being located near where the forge welding occurs.
In order for the above-described method to result in a high quality bond, both the fin and the tube must be at the proper temperature for forge welding when they come into contact. As the surface of the tube is broad relative to the surface of the bottom edge of the fin, the length of the high frequency weld leg on the tube, i.e., the distance between the electrical contact at the tube and where the forge welding occurs, is desirably adjusted to achieve the proper temperature for forge welding. Adjustment of the weld leg length to create the proper temperature can be difficult when the material configurations to be bonded are extremely different and when physical constraints interfere.
Such a difficult situation can exist in the welding of a fin to a tube. If the electrical contact on the rotating tube is located too close to where the forge welding occurs, the short resistance path may not produce enough concentrated heat to create the proper temperature in the tube for forge welding. If the electrical contact on the tube is located over about 60.degree. from horizontal, the contact may not sense the opposing current on the fin, and thus produce no concentrated heat.
Attempts to solve this problem by slowing down the welding process or increasing weld power have resulted in excess power, i.e., temperature, being produced in the relatively thin fin material. The excess temperature causes the metallic fin to become soft or even molten, which condition is not conducive to welding because the fin is the mechanical element providing the forging pressure.
Another difficulty with high frequency welding occurs in the welding of stainless steel fins to a chrome alloy tube. The oxides of chrome, which are present on the surface of a chrome alloy tube, have a higher melting temperature than the parent material. This is detrimental to the weld process since the heat created by resistance to electrical current flow creates a soft under material of the parent metal while leaving a substantially firmer crust of chrome oxide. The chrome oxide crust retards bonding of the fin to the tube surface under the forging process.
A general object of the present invention is to provide an improved method for welding a metallic fin to a metallic tube. A more specific object of the present invention is to provide an improved method for high frequency welding of a fin to a tube. Further objects and advantages of the present invention will be made clear by the following specification and claims.
A preliminary search was conducted on the present invention and the following represent the results of the search.
______________________________________ PATENTEE DATE ______________________________________ U.S. PAT. NO. 3,214,563 Ford 10/26/65 3,278,720 Dixon 10/11/66 3,340,601 Garibotti 09/12/67 4,167,662 Steen 09/11/79 4,507,540 Hamasaki 03/26/85 4,829,153 Correy 05/09/89 4,841,617 Schmidt, et al. 06/27/89 4,891,491 Duley, et al. 01/02/90 5,006,688 Cross 04/09/91 5,006,694 Handke, et al. 04/09/91 JAPANESE PATENT 63-30193 Takashi Oomae 02/08/88 63-295066 Takashi Terasawa, et al. 12/01/88 62-263869 Kazuo Azuma 11/16/87 61-3682 Shiyouzou Satou, et al. 01/09/86 61-232079 Moriaki Ono, et al. 10/16/86 59-82187 Tsutomu Toyohara, et al. 05/12/84 59-232690 Sadao Sugiyama, et al. 12/27/84 58-184085 Taizou Nakamura,et al. 10/27/83 ______________________________________
U.S. Pat. No. 3,214,563 relates to utilizing an optical laser to accurately position a hole in conductive material by establishing, pulsing, and/or continuing a hot plasma at the point where a hole is to be drilled with an electric arc. The arc is established after the plasma is formed by the laser.
U.S. Pat. No. 3,278,720 relates to welding.
U.S. Pat. No. 3,340,601 is directed to the fabrication of alloy-diffused junction semiconductor devices. The fabrication method includes preheating a chip by directing a beam from a laser through a glass port onto the chip prior to and during the electron beam impingement.
U.S. Pat. No. 4,167,662 relates to a method for cutting, welding, drilling, or surface treating a workpiece, the method comprising directing a laser beam towards a workpiece and striking an arc between an electrode and the heat affected zone created by the laser beam in the workpiece.
U.S. Pat. No. 4,507,540 relates a welding method which combines MIG welding and laser welding.
U.S. Pat. No. 4,829,153 relates to an improved inert gas shielded tungsten arc welder, wherein the improvement comprises in combination with the tungsten electrode, a starting laser focused upon the tungsten electrode which to ignite the electrode heats a spot on the energized electrode sufficient for formation of a thermionic arc.
U.S. Pat. No. 4,841,617 relates to welding heat exchanger fins to a heat exchanger tube with a laser beam that is directed toward the region where the strip runs onto the surface of the tube.
U.S. Pat. No. 4,891,491 relates to enhancing the efficiency of laser processing of metals by preprocessing with another laser so as to define a track of reduced reflectivity.
U.S. Pat. No. 5,006,688 relates to laser-arc welding.
U.S. Pat. No. 5,006,694 relates to a laser welding operation.
Japanese Patent 63-30193 relates to welding thick plates at a high speed with a medium output laser by performing the deep penetration welding on the surface side of the plate and, afterward, performing the MIG welding on its rear side.
Japanese Patent 63-295066 relates to welding sheet metals with high quality at high speed by performing the laser beam irradiation under specific conditions in advance on the objective position of a W electrode at the time of performing the TIG welding of the sheet metals with the specific thickness.
Japanese Patent 62-263869 relates to improving starting properties and directivity of an arc under welding conditions with a wide range by spot-projecting laser beams on metal materials to be welded to generate heat thereon and applying an electric current between the heat generating part and an electrode of a TIG welding machine to generate the arc.
Japanese Patent 61-3682 relates to executing defectless and satisfactory welding with high efficiency by using a specific laser beam as a preceeding heat source and defocussing specifically the spot diameter of a laser beam.
Japanese Patent 61-232079 relates to obtaining a broad bead on the rear side of materials to be welded by irradiating an arc of TIG or plasma welding onto the weld line on the rear side of the surface to be irradiated with a laser and moving simultaneously the laser beam and arc, thereby welding the materials.
Japanese Patent 59-82187 relates to strengthening metal deposited at a final pass by irradiating an arc to the deposited material.
Japanese Patent 59-232690 relates to performing high quality welding by welding simultaneously the joint part of both members with a heat source of high energy density such as a laser from the outside circumferential side of said part, and with an arc heat source from the inside circumferential side.
Japanese Patent 58-184085 relates to increasing efficiency of utilization of laser beam energy and to performing more uniform welding of high quality, by heating the back side of the weld zone supplying plasma forming gas to the vicinity of the weld zone and melting added metal.
None of the above-mentioned patents describe an improved method of high frequency welding of a fin to a tube.